There have hitherto been disclosed some technologies regarding a method for producing a glass material and a method for producing an optical member for EUV lithography.
For example, JP-A-2008-115054 (Patent Document 1) discloses a method for producing a titania-silica glass which is used as an optical member such as a photomask and a mirror of extreme ultraviolet lithography (EUV lithography) in a manufacturing process of semiconductors, liquid crystals or the like.
Specifically, Patent Document 1 discloses a method for producing a titania-silica glass, comprising feeding a gas prepared by vaporizing a liquid raw material of a silica source and a gas prepared by vaporizing a liquid raw material of a titania source from separate tube selected from a first introduction tube which is center of a multiple tube of a concentric multitubular burner, a second introduction tube adjoining the periphery of the first introduction tube, and a third introduction tube adjoining the periphery of the second introduction tube; hydrolyzing the gases in an oxyhydrogen flame projected from the burner; depositing glass fine particles on a target; and then subjecting the glass fine particles to a thermal vitrification treatment.
Also, Patent Document 1 discloses that the gas prepared by vaporizing a liquid raw material of a silica source is fed from the first introduction tube, and the gas prepared by vaporizing a liquid raw material of a titania source is fed from the third introduction tube sandwiched between an oxygen introduction tube and a hydrogen introduction tube; and that hydrogen is fed from the second introduction tube, and oxygen is fed from a fourth introduction tube adjoining the periphery of the third introduction tube.
Here, in the conventional production method of a titania-silica glass as disclosed in Patent Document 1, for example, the flow rate of the gas prepared by vaporizing a liquid raw material of a silica source and the flow rate of the gas prepared by vaporizing a liquid raw material of a titania source are generally set to values larger than the flow rate of the hydrogen gas and the flow rate of the oxygen gas. According to this, diffusion of the gas prepared by vaporizing a liquid raw material of a silica source and the gas prepared by vaporizing a liquid raw material of a titania source by the hydrogen gas and the oxygen gas is reduced. As a result, it is considered that it becomes possible to enhance a yield of the titania-silica glass.
However, with respect to the titania-silica glass produced by the conventional production method of a titania-silica glass, in some case, uniformity of the concentration distribution of titanium in the titania-silica glass was low.
Also, the concentration distribution of titanium in the titania-silica glass correlates with the distribution of a coefficient of thermal expansion of the titania-silica glass. For that reason, in some case, uniformity of the distribution of a coefficient of thermal expansion of the titania-silica glass produced by the conventional production method of titania-silica glass was low.
Meanwhile, when the titania-silica glass is used as a glass substrate of an optical member of EUV lithography, the distribution of a coefficient of thermal expansion of the titania-silica glass is required to have higher uniformity. For that reason, when the titania-silica glass is used as a glass substrate of an optical member of EUV lithography, in some case, uniformity in the distribution of a coefficient of thermal expansion of the titania-silica glass produced by the conventional production method of a titania-silica glass was insufficient.
Patent Document 1: JP-A-2008-115054